An active matrix type liquid-crystal display device has a structure in which pixel circuits that include a liquid-crystal capacitor and a writing control thin film transistor (TFT) are arranged in a two-dimensional form. In a liquid-crystal display device in the related art, the TFT is formed using, for example, amorphous silicon. In recent years, a technology for forming the TFT using an oxide semiconductor such as indium gallium zinc oxide (IGZO) has been developed. The TFT formed using an oxide semiconductor is characterized in that a leakage current during OFF time is smaller than that in the TFT in the related art. Therefore, according to the liquid-crystal display device in which the TFT is formed using the oxide semiconductor, it is possible to improve the display quality from that in the liquid-crystal display device in the related art.
In the liquid-crystal display device, there is a problem in that an afterimage, a ghosting, and a flickering occur on a display screen. When a DC voltage is applied to a liquid-crystal layer, a movement of charge due to ionic conductivity is induced in the liquid-crystal layer, and the moved charge is accumulated on the alignment film that is applied on electrodes. When the voltage caused by the charge accumulated on the alignment film that is applied on the electrodes is applied between the electrodes (liquid-crystal layer), afterimages and ghosting occur. In the related art, in order to prevent afterimages or ghosting, the liquid-crystal display device performs an AC drive (polarity inversion drive) in which the polarity of the voltage written into the pixel electrode is switched at a predetermined interval. In the AC drive, when the polarities are positive and negative, the same amounts of voltage (the voltages of which the absolute values are the same) are applied. However, even when the same amounts of voltage are applied at the time of the polarities are positive and negative, the electric potentials (absolute values) between the electrodes are slightly different due to an affection of the load such as TFT characteristics or a panel wiring. For this reason, a difference corresponding to the period of the polarity inversion appears in the display luminance, and thus, a deterioration of display quality called a flickering occurs. In the related art, in order to prevent the flickering, the liquid-crystal display device has a function of adjusting the level of the voltage written into the pixel electrode for each polarity.
In addition, in the liquid-crystal display device, when the power is cut off, there is a problem that unnecessary charge remains in the pixel electrode, and thus, afterimages and ghosting occur. In PTL 1, as a method for preventing the afterimage in the liquid-crystal display device, a method is disclosed, in which, when the power is cut off, the voltages in all the gate lines are controlled such that they are in a level that the writing control TFT becomes temporary in ON state. Specifically, as illustrated in FIG. 10, a transistor Tr1 is provided corresponding to a gate line 91, and a power source voltage and voltages of control lines VH1 and VH2 are changed as illustrated in FIG. 11. In this way, when the power is cut off, the transistor Tr1 is controlled such that they are in ON state, the voltage in the gate line 91 is a high level, the writing control TFT 92 in the pixel circuit is in ON state, and thus, the charge remaining in the pixel electrode 93 can be discharged via a source line 94.